In general, linear polarizers are constructed in a fashion such that a series of long thin conductors, placed parallel to each other, acts as a polarizer. This technique works well provided the length of the conductors is of the order of magnitude of the wavelength of the light to be polarized and their width and spacing is considerably less than that. When electromagnetic waves are incident on such a device, the component whose electric field oscillates parallel to the conductor length will set up currents in the conductor and be reflected or absorbed, whereas the transverse component will be affected only slightly and transmitted through the material thus providing polarized light.
The particular type of polarizer of present interest consists essentially of polyvinyl alcohol (PVA) sheets with the alignment of the polymer chains accomplished by stretching. Long needle shaped crystals of iodine are absorbed in the stretched sheet attaching themselves at the appropriate sites along each polymer chain and thus form the elongated regions of electric conductivity required for the absorption of light. By proper control of the iodine staining process, the polarizer can be made effective over a spectral region extending from 300 to 700 nanometers (nm).
The PVA film/iodine type of polarizer is selected for use in flat panel displays because it offers superior light blocking characteristics when operated in the crossed configuration and higher transmission when operated in the parallel configuration as compared to other birefringent crystal linear polarizers. However, in this type of polarizer, a "blue-leak" occurs at 428 nm. All but a very small amount of this one wavelength of light is passed by the PVA/iodine polarizers whether they are parallel ("clear") or crossed ("black"). The only 428 nm light that is absorbed is done so in the cellulose acetate laminates that are used to encapsulate the polarizing materials. In the crossed configuration (off-state), the 428 nm leak is highly undesirable and accounts for greater than 90% of the total amount of light that is not suppressed by the polarizers. Non-neutral off-state color experienced in a flat panel display that uses a PVA film/iodine based polarizer is due to the physical inability of the iodine to absorb light emitted at 428 nm regardless of its field orientation parallel or perpendicular.
At the long wavelength end of the visible spectrum, the polarizers in the crossed configuration (off-state) become increasingly transmissive to the longer wavelengths of visible light (still providing good attenuation for all but the longest of red wavelengths) until they become completely invisible to near infra red. In general, PVA/iodine polarizers used in the crossed configuration are very good attenuators of light that falls in the middle of the visible light spectrum. Left unchecked, the combination of the blue and red leakage elevates the background luminance of a flat panel display. The consequences are that contrast and dimming ratio are reduced.
In addition to the quantity of light that is leaked through the crossed polarizer configuration, the background color in the off-state is not the neutral grey or black color that is intended and the display emits a purple hue. This is of particular concern where requirements levied against the display unit demand the "black" background color be the same color as the emitted "white" light. Adjusting the emitted white color to a purple hued white has catastrophic effects on luminance. Under this condition, a very large loss in luminance occurs because the color purple, a mix of blue and red leaked by the polarizers in the crossed configuration, lacks the green wavelengths of light where the human visual system is most efficient.
Furthermore, liquid crystal materials (primarily the glass substrates) have been experimentally proven to fluoresce in the near infra red spectrum when exposed to very short wavelength visible and ultra violet (UV) light. Fluorescence of liquid crystal materials in the near infra red region have the potential of reducing the effectiveness of night vision goggles that are extremely sensitive to emissions in the near infra red spectrum.
It is also well-known that the human visual system becomes much more sensitive to blue and violet light when night adapted (the so-called Purkinje effect). This has the effect that the bluish or purplish background of an LCD is much is much more noticeable when operated at night, causing a further reduction of apparent contrast.
Therefore, there is a need for an active matrix liquid crystal display flat panel that optically eliminates background luminance. There is a further need to increase the contrast and dimming ratio and provide Night Vision Imaging System (NVIS) compatibility.